The proposed studies trace critical events of development of reciprocated connections between polymorphic neurons of neocortical fields and class I (specific) projections of corresponding thalamic nuclei in normal and reeler mutant mice. Hodologic methods, based upon anterograde and retrograde transport of horseradish peroxidase (HRP), will be used in animals at a range of developmental ages from embryonic day 15 (E15) to postnatal day 0 (P0). Experiments will examine the hypotheses: 1. that the polymorphic cells, the principal neuronal population in the developing neocortex at E15-17, are a sufficient population to establish thalamic nucleus - neocortical field interrelationships and 2. that thalamic axons reach the cortex as early as E15 and that this is before the cortical axons reach the thalamus. Correlated light and electron microscopic analyses of single thalamocortical axons impregnated by the Golgi method or labelled by HRP histochemistry will define patterns of arborization and termination of identified thalamocortical axons and will identify synaptic specializations if present. To the extent that connections are formed in the same sequence in normal and reeler embryos, in which neocortical cell position is abnormal, the developmental sequence is not critically dependent upon cell position within the cortex.